Solid waste materials of the type exemplified by municipal waste have traditionally presented problems of disposal which have become increasingly critical in recent years as a result of not only a rapidly increasing population but the compounding difficulty of a drastic increase in per capita production of solid waste. Conventionally, such solid waste has been disposed of by such means as incineration and landfill. Obviously, with the ever increasing concern with problems of natural resources and the dwindling supply of acreage suitable for landfill operations within a reasonable distance of population centers, both of these methods of solid waste disposal are becoming less acceptable.
While proposals have been made from time to time for processing solid waste material so that the various components thereof can be reused, a continuing problem with such proposals is the quality of the recovered material, which is often so inferior as to reduce the market value of the recovered components below that which would make such recovery operations commercially attractive or even economically feasible. For example, although reclaimed glass has a potential value for reuse as cullet in the manufacture of new glass products, a commercially feasible method of reclaiming glass from waste materials in a form suitable for such reuse has eluded the industry prior to the invention disclosed and claimed in my above U.S. Pat. No. 3,720,380.
Another method of sorting refuse to isolate reclaimable glass is disclosed in U.S. Pat. No. 3,650,396 to Gillespie et al, in which the initial steps are the same as in my U.S. Pat. No. 3,720,380, in that the refuse or trash is pulped, and passed through a cyclone separator to separate the inorganics from the water and fiber. The separated inorganics are screened to isolate particles in the size rangee of between 1/4 inch and 3/4 inch (which screening allows the particles to be subsequently optically sorted), dried, and passed through a series of pneumatic separators to separate the glass particles from the lighter weight particles. Since the object of U.S. Pat. No. 3,650,396 is to isolate the reclaimable glass, the lighter fraction produced by the separators, which are operated by passing a high volume of air upward through the columns so that the particles will separate by gravity, includes all of the lighter nonglass particles without distinction but including light weight metals, stones, bones, wood, rubber and plastics.
As is pointed out in U.S. Pat. No. 3,650,396, in addition to glass, municipal waste contains substantial quantities of both ferrous and non-ferrous metals, particularly aluminum in several forms. For example, aluminum in an alloy form is used in many cans, can tops, and containers for food, especially as trays for convenience foods such as "TV Dinners" and the like. Large quantities of relatively pure aluminum occur in solid waste as pieces of foil in a variety of thicknesses, and a third relatively common form comprises structural parts cast from aluminum, such as lawn furniture and the like.
The value of reclaimed aluminum is relatively high, and it is desirable to facilitate its recovery in as uncontaminated form as possible in order to obtain the maximum return. While U.S. Pat. No. 3,650,396 discloses that a lighter fraction including aluminum can be isolated in addition to the glass, it would not be useful in and of itself to recover all of the types of aluminum because that does not isolate aluminum alone. It is able only to isolate aluminuum along with other lightweight odds and ends. Thus to isolate a particularly valuable type of aluminum would be even further removed from the teachings of U.S. Pat. No. 3,650,396.
What is more important in the background of the present invention is that the several forms of aluminum differ among themselves in value, with the first form noted above being of significantly greater value, for a number of reasons. In particular, aluminum as used in cans and other food containers, which for purposes of distinction will be referred to herein as "wrought" aluminum, has a definite and uniform alloy content, and recovered scrap aluminum of this type can be reused for similar purposes with a minimum of processing expense. The value of this form of recovered scrap, however, depends upon its relative purity, in terms of contamination both by other forms of aluminum and by other materials.